Andy, does your 6x6 camera have a darkslide per chance (like on a Hasselblad film back for instance)? If so, you could pull it out in increments, giving a -5 exposure on a white/gray ground each time you pull it out. If you can pull it out 11 times in the length of 6cm, dividing the film into 11 sections, the first exposed portion will eventually get +5 exposure and the intermediate sections will get +4, +3, +2, +1, 0, -1, -2, -3, -4, -5.

You could do the same thing under an enlarger, but it'd just be a bit trickier.